CCAudio.js a finished playback callback is needed

[boqugames]

Add function codes to cc.Audio class

setFinishCallback: function (cb) {
    if (!this._element) return true;
    this._element.finishedCallback(cb);
},

and add function codes to cc.Audio.WebAudio.prototype

finishedCallback: function (cb) {
    var loop = this._loop;
    this._currentSource.onended = function() {
        if (!loop) {
            cb();
        }
    }
}